With the rack in the middle position, preheat the oven to 180 °C (350 °F). Generously butter a 23-cm (9-inch) tube pan, plain or fancy (see picture).
In a bowl, combine the flour, cocoa powder, baking powder and baking soda. Set aside.
In another bowl, combine the milk and sour cream. Set aside.
In the third bowl, cream the butter with the sugar until smooth. Add the vanilla and the eggs, beating constantly until the mixture is smooth and creamy.
Stir in the dry ingredients alternately with the milk mixture. Pour the batter into the prepared pan. If you use the "pine forest" tube pan, you must place it on a baking sheet for stability during baking. Bake for 50 minutes or until a toothpick inserted in the centre of the cake comes out clean. Cool slightly before unmoulding, then cool completely on a rack.
Just before serving, dust the cake with icing sugar. Serve with chocolate sauce (see recipe).